Instrument transformers are used to step-down current or voltage to measurable values, and are widely used in various power systems. There are two basic types of instrument transformers, i.e. voltage transformer and current transformer. A voltage transformer (VT, in the description also including Capacitive Voltage Transformer, CVT) is designed to reproduce accurately the primary voltage on the secondary side, while a current transformer (CT) is for current signals. The correct information about the primary values of voltages (from VT) and currents (from CT) are the basis for the correct operation of an intelligent electronic device (IED). However, failures may happen in the secondary circuits between the VT and the IED (this condition is referred to as VTfailure in the description) or between the CT and the IED (this condition referred to as CTfailure in the description), and result in unwanted operations of various protection functions of the IED. For example, VTfailure may cause a distance protection function to mal-operate. On the other hand, in the case of a CTfailure, a ‘false’ differential current will appear on the phase of CTfailure, and if the magnitude of this ‘false’ differential current is bigger than a startup threshold, the differential protection function may mal-operate. Furthermore, single-phase or double-phase CTfailure will result in a ‘false’ negative sequence current or zero sequence current, which will cause unwanted operations of negative/zero sequence current based protections. In order to avoid unwanted operations that may be caused by CTfailure or VTfailure, reliable, sensitive and high-speed Current Circuit Supervision and Voltage Circuit Supervision (also named as Fuse Failure Supervision) have always been very important.
Many methods for current/voltage circuit supervision have been developed. Several conventional solutions of current/voltage circuit supervision will be described below.
A) Current Circuit Supervision Methods
One current circuit supervision method is to compare the zero sequence current from a three phase set of current transformer windings with the neutral point current on a separate input taken from another set of windings on the current transformer. A detection of a difference indicates a fault in the circuit, and is used as alarm or to block protection functions expected to give unwanted tripping. This method needs customers to mix current circuits from different windings or different current transformers. Consequently this method increases the complexity of the hard wiring, and thus decreases the reliability of the IED. Furthermore, this method can not detect 3-phase CTfailure because under such conditions, there will be no zero sequence current in both windings or both current transformers.
Another current circuit supervision method is implemented by checking the presence of zero sequence current and zero sequence voltage. A high value of residual current without the presence of zero sequence voltage indicates CTfailure condition. However, this method can not detect 3-phase CTfailure because it is based on zero sequence measurements. Secondly, it can not work properly during asymmetrical operation condition (e.g. single-pole reclosing period) because there are always both zero sequence current and zero sequence voltage. Thirdly, it may mal-operate if the zero sequence impedance of the system is very small. Moreover, sometimes the voltage inputs are even not available.
Another current circuit supervision method is to detect the sudden disappearance of a phase current. However, when a fault occurs on a line that connects the power source and the load, the protection IED on the load side may also detect the sudden disappearance of the phase current, thus the CTfailure supervision function will operate incorrectly.
Another current circuit supervision method is based on changes in current flows connecting with the same busbar. Its theory is that internal or external faults will cause changes in at least two current flows, whereas a CTfailure only affect a single current flow. This method, however, requires sampled data from all the power system components (lines, transformers, etc) that are connected to the protected busbar, which means large amount of communication traffic and slow operation speed.
B) Voltage Circuit Supervision Methods
One Voltage circuit supervision method is to compare the measured zero sequence voltage under normal operating condition with a preset threshold. If the zero sequence voltage is bigger than the threshold, VTfailure will be detected. This method is useful only when there is no disturbance or fault, and thus a reliable start-up element that can detect disturbance or fault is needed. Furthermore, it is not applicable during asymmetrical operation conditions (e.g., single-pole reclosing period).
Another voltage circuit supervision method is implemented by checking the presence of zero/negative sequence current and zero/negative sequence voltage. A high value of zero/negative sequence voltage without the presence of the zero/negative sequence current indicates a VTfailure condition. This method can not detect 3-phase VTfailure. It can not work properly during unsymmetrical operation condition because there are always both zero/negative sequence current and zero/negative sequence voltage.
Another Voltage circuit supervision method is implemented by comparing the measured voltages from two separate sets of secondary windings of the voltage transformer. If the measured voltages of the same phase from two separate secondary windings of the voltage transformer are different, a VTfailure condition will be determined. This method needs measured voltages from two sets of secondary windings, which increases the complexity of the hard wiring, and thus decreases the reliability of the IED.
Furthermore, the above mentioned current/voltage circuit supervision methods based on both voltages and currents will fail if failures occur simultaneously on both current and voltage secondary circuits.
As shown in the above description, the existing methods either are unreliable under certain conditions or rely on large amount of communication.
Since the output of current/voltage circuit supervision functions are used to block relative protection functions (e.g., detection of VTfailure will block distance relay, while detection of CTfailure will block current differential protection) in order to avoid unwanted operations that otherwise may occur, the supervision functions must be reliable, sensitive and have short operate time to prevent unwanted operations from fast-acting, sensitive protection functions in case of failures in the current/voltage secondary circuits.